For measuring a target point, numerous geodetic surveying apparatuses have been known since ancient times. In this case, direction or angle and usually also distance from a measuring apparatus to the target point to be measured are recorded and, in particular, the absolute position of the measuring apparatus together with reference points possibly present are detected as spatial standard data.
Generally known examples of such geodetic surveying apparatuses include the theodolite, tachymeter, total station and also laser scanner, which are embodied in the terrestrial and airborne variants. One geodetic measuring device from the prior art is described in the publication document EP 1 686 350. for example. Such apparatuses have electrical-sensor-based angle and, if appropriate, distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the apparatus and, if appropriate, also have to be combined with an external reference system for absolute position determination.
Modern total stations have microprocessors for digital further processing and storage of detected measurement data. The apparatuses generally have a compact and integrated design, wherein coaxial distance measuring elements and also computing, control and storage units are usually present in an apparatus. Depending on the application, total stations are additionally equipped with motorization of the targeting or sighting device and—in the case of the use of retroreflectors (for instance an all-round prism) as target objects—means for automatic target seeking and tracking. As a human-machine interface, the total station can have an electronic display control unit—generally a microprocessor computing unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data detected in an electrical-sensor-based manner are fed to the display control unit, such that the position of the target point can be determined, optically displayed and stored by the display control unit.
For sighting or targeting the target point to be measured, geodetic surveying apparatuses of the generic type such as total stations comprise a telescopic sight, such as e.g. an optical telescope, as sighting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilting axis relative to a base of the measuring apparatus, such that the telescopic sight can be aligned with the point to be measured by pivoting and tilting. Modern apparatuses can have, in addition to the optical viewing channel, a camera for detecting an image, said camera being integrated into the telescopic sight and being aligned for example coaxially or in a parallel fashion, wherein the detected image can be represented, in particular, as live image on the display of the display control unit and/or on a display of the peripheral apparatus—such as e.g. the data logger—used for remote control. In this case, the optical system of the sighting device can have a manual focus—for example an adjusting screw for varying the position of a focusing optical system—or an autofocus, wherein the focus position is varied e.g. by servomotors. By way of example, such a sighting device of a geodetic surveying apparatus is described in EP 2 219 011. Automatic focusing devices for telescopic sights of geodetic apparatuses are known e.g. from DE 197 107 22, DE 199 267 06 or DE 199 495 80. Moreover, the construction of generic telescopic sights of geodetic apparatuses is disclosed by way of example in the publication documents EP 1 081 459 or EP 1 662 278.
On account of the beam path that is usually to be utilized jointly both as viewing channel and for measurements, such apparatuses require the technical design of said beam path in the manner of construction of a telescope with specialized, high-precision optical systems that are to be produced with a high outlay. Furthermore, an additional separate transmitting and receiving channel and also an additional image plane for the wavelength of the distance measuring device can be provided for the coaxial electronic distance measurement.
Since target objects (e.g. the plumb rods with target mark, such as an all-round prism, which are usually used for geodetic purposes) cannot be targeted sufficiently precisely with the naked eye on the basis of the sighting device despite the 30-fold optical magnification often provided, conventional surveying apparatuses in the meantime have as standard an automatic target tracking function for prisms serving as target reflector (ATR: “Automatic Target Recognition”). For this, a further separate ATR light source—e.g. a fiber-coupled laser diode, which emits electromagnetic radiation having a wavelength preferably in the infrared range of 850 nm, for example—and a specific ATR detector (e.g. CMOS area sensor) sensitive to said wavelength are conventionally additionally integrated in the telescope.
In the context of the ATR fine targeting function, in this case the ATR measurement beam is emitted in the direction of the optical targeting axis of the sighting device and is retroreflected at the target reflector and the reflected beam is detected by the ATR sensor. Depending on the deviation of the alignment of the optical targeting axis from the prism, in this case the impingement position of the reflected radiation on the ATR sensor also deviates from a central sensor area position (i.e. the reflection spot of the ATR measurement beam retroreflected at the prism on the ATR area sensor does not lie in the center of the ATR area sensor and therefore does not impinge on a desired position defined e.g. on the basis of calibration as that position which corresponds to the optical targeting axis).
If this is the case, then the alignment of the sighting device is slightly readjusted in a motorized manner in such a way that the ATR measurement beam retroreflected at the prism impinges highly precisely in the center of the sensor area on the ATR area sensor (i.e. the horizontal and vertical angles of the sighting device are thus iteratively changed and adapted until the center of the reflection spot coincides with the desired position on the ATR area sensor). Alternatively, a residual deviation between the impingement point of the retroreflected ATR measurement beam on the ATR area sensor and the center of the sensor area can also be taken into account computationally and converted into an angle, which is correspondingly added to the solid angle—detected with the aid of the angle sensors—at which the targeting axis points.
Besides the ATR fine targeting function, an automatic target tracking functionality can also be provided in a similar manner and using the same ATR components (such as ATR light source and ATR detector).
An impairment of the distance measurement or of the automatic target tracking can be caused here e.g. by stray radiation, wherein, besides the radiation having a defined characteristic that is emitted by the radiation source, further disturbing radiation components can impinge on the detector (for detecting the radiation that is emitted by the radiation source and reflected) and cause a deviation of a measured value.
Ambient or extraneous light can thus lead to undesired influences or impairments of measuring devices. Direction or position measurement by means of optical semiconductor sensors (e.g. PSD) or area sensors (e.g. CCD or CMOS) can be corrupted e.g. by parasitic background light impinging on the detector. This leads for example to centroid shifts, induced by a modified light distribution on the sensor or by a variation of the electronic operating point of the sensor.
Measuring devices comprising spectrally narrowband radiation sources such as laser diodes in some instances afford the possibility of suppressing the ambient light on the receiving channel for the most part by means of an appropriate cut-off filter or bandpass filter. Primarily lasers emit in a very narrowband manner in a spectral range of less than 1 nm. However, cost-effective lasers such as laser diodes have the disadvantage that the center wavelength thereof differs from specimen to specimen; moreover, the emission wavelength in most semiconductor laser diodes is dependent on the operating point (forward current) and the chip temperature.
This has the effect that the optical bandpass filters, generally configured as interference filters, upstream of the detector are of spectrally wide design. As a result, however, the ambient light cannot be optimally suppressed.
This disadvantage is analogously applicable to coordinate measuring machines comprising optical sensors for measuring coordinates of measurement points.
Coordinate measuring machines, for checking technical components with regard to their form and their dimensions, in this case have a movable measuring head that can be moved within a measurement space. For this purpose, as is known, coordinate measuring machines can be constructed as gantry constructions or articulated arms by means of which the measuring head is freely movable in three directions (X,Y,Z) and wherein the position of the measuring head and thus a position on a workpiece measured optically by the measuring head can be determined in the measurement space continuously and precisely.
Furthermore, the impairment mentioned above can occur in the case of contactlessly measuring optical scanners for generating e.g. a three-dimensional representation of a surface of an object. These scanning devices are usually designed as laser scanners, wherein laser radiation having a defined wavelength is emitted and reflected at the object. By means of a detector, which in the optimum case detects and measures exclusively light having the same wavelength, distance and direction measurements can thus be carried out in a point-resolved manner. In this case, the laser beam can be guided line by line over the surface.
Distance measuring modules comprising a plurality of light sources having different emission wavelengths are also known, as can be gathered from DE 198 40 049. for example. There, depending on the measurement application, either one or the other laser is directed at the same common detector; in that case, the interference filter upstream of the receiver is designed as a double interference filter having two transmission windows. This has the disadvantage that the ambient light is not optimally suppressed. In the case of multi-channel transmitting units which differ spectrally-optically and are directed at a common receiving unit, the problem of suppressing stray radiation and also of mutual channel crosstalk increases with the number of channels.
The prior art discloses at least two approaches for solving the stray radiation problem. Firstly, a filter is arranged into the beam path of the receiving optical system upstream of the detector in such a way that exclusively electromagnetic radiation in a defined wavelength range—as narrowband as possible—can pass through the filter and impinge on the detector. As is known, the filter used for this purpose has a predetermined transmission behavior. Moreover, the radiation source is electronically and thermally stabilized with regard to the emitted wavelength with a high outlay, i.e. is supplied with a constant voltage and current and operated at a defined temperature, in particular by the cooling or heating of the radiation source. This specified emitting-receiving device requires a high level of constructional outlay and energy expenditure.
The second approach proposes the use of two filters, one high-pass filter and one low-pass filter, wherein the filters are arranged one behind another in the beam path and thus achieve a relatively well-defined filter effect. The filters can additionally be tilted relative to the beam path thereby enabling a change in the filter behavior and thus an adaptation to the incoming radiation. Such an arrangement can be gathered from DE 101 21 288. for example. What is disadvantageous about this arrangement is the occurrence of polarization effects during the tilting of the filters.
The two solution approaches additionally exhibit the disadvantage of a highly complex and structural-space-intensive construction of the respective measuring apparatuses. Moreover, very complex control or regulation of the radiation source or of the tiltable filters is required in order that optimum detection in the suitable wavelength can be ensured continuously.